The present invention relates to the detection of obstacles to the horizontal flight of helicopters, and more particularly, to a pulsed doppler radar system whose antenna is mounted near the tip of a rotor blade and that processes the radar echos' Doppler shift produced by the tangential velocity of the blade tip antenna in such a way as to discriminate against echos arriving from above or below the rotation plane of the blades, thus achieving a synthetically narrowed antenna beam that enables the detection of small flight path obstacles in the presence of ground clutter from terrain objects below (or above) the helicopter.
The helicopter's capability for operation close to the ground and, with its hovering capability, operating in propinquity to obstructions such as buildings, trees, etc., is severely hampered under conditions of low visibility. In order to move safely in a horizontal plane, the pilot must be aware of all obstacles at his altitude. One of the most dangerous types of obstacle is horizontal wires. These are quite prevalent in civilized countries and are extremely difficult to see even under conditions of good visibility. A short range navigation aid is needed to indicate to the helicopter pilot the relative position of such obstacles and to alert him to a threatening situation.
Radar is, in many respects, an ideal sensor, with its ability to see through clouds and darkness and even to penetrate rainfall. However, distinguishing the echos from an object such as a horizontal wire in the presence of the reflections from large areas of terrain around the helicopter poses a difficult problem for the radar designer.
One approach to detecting obstacles in the flight plane is to discriminate against terrain echos on the basis of vertical antenna directivity, whereby echos from obstacles outside the flight plane are attenuated compared to those close to the flight plane. Unfortunately, directivity in simple radar systems requires large antenna apertures, which are heavy and not compatible with the aerodynamics of airborne vehicles. However, it is well known that a large synthetic aperture can be generated by processing signals received while a relatively small physical antenna is transported through space. This principle is exploited in some airborne radars in which special signal processors use the Doppler history of reflections from terrain objects imparted by aircraft motion to enhance the horizontal resolution of the physical antenna. However, these techniques fail when the airborne vehicle velocity approaches zero. Furthermore, these systems do not enhance resolution in the vertical plane, as required in this application.
The present invention achieves synthetic aperture generation by utilizing the tangential velocity of the tip of a helicopter rotor blade. Echos received by a transmit-receive antenna mounted thereon are Doppler shifted in frequency because of the rotor tip velocity, with the highest Doppler frequency shift being observed for objects that are in line with the tangential velocity vector. By using pulse Doppler radar techniques, it is possible to reject echos from objects that are not in line with the antenna's velocity vector to such an extent that the otherwise large ground clutter echos are reduced sufficiently that small echos from objects such as horizontal wires are detectable.